ci: update nightly and fail on doc warnings (#92)

Currently, CI testing uses a fixed nightly toolchain. While this is
useful against a spurious nightly build, the repository already uses the
latest nightly for formatting, lints, and checks. I think it's worth it
to stick with the latest nightly across the board.

Additionally, this updates documentation checks to fail on warnings.
This ensures that the documentation warnings don't go unnoticed.

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         rust:
           - stable
-          - nightly-2023-06-04
+          - nightly
     steps:
       - name: checkout
         uses: actions/checkout@v4
@@ -25,4 +25,4 @@ jobs:
       - name: test/release
         run: cargo +${{ matrix.rust }} test --release --all-features
       - name: Build documentation
-        run: cargo +${{ matrix.rust }} doc --no-deps
+        run: RUSTDOCFLAGS="-D warnings" cargo +${{ matrix.rust }} doc --no-deps

src/protocols/scalar_protocol.rs

@@ -13,7 +13,7 @@ pub trait ScalarProtocol {
     /// Returns a non-zero random Scalar
     fn random_not_zero<R: RngCore + CryptoRng>(rng: &mut R) -> Scalar;
 
-    /// Construct a scalar from an existing Blake2b instance (helper function to implement 'Scalar::from_hash<Blake2b>')
+    /// Construct a scalar from an existing Blake2b instance (helper function to implement `Scalar::from_hash<Blake2b>`)
     fn from_hasher_blake2b(hasher: Blake2bMac512) -> Scalar;
 }